Well you already have Double Grape and 3BOG seeds. With the Double Grape 3BOG cross... you might get phenos that are strong representation of both together or may lean towards 1 or the other... and you still keep your own separate strains. If you want to make more double grape seeds or 3BOG seeds you can... or if you like the crosses you can make your own cross seeds.
Also, you can fill up the tent with a few of these. They shouldn't get too big.